The present invention relates to an arc-spraying filler wire by means of which coatings highly resistant to mechanical and/or chemical wear can be produced.
Places in which both mechanical and chemical wear is very heavy, for which reason the apparatus must be maintained or repaired frequently, include peat power stations, in which the boiler walls, grates, feeder screws, flue gas blowers and other similar parts are exposed to heavy chemical and mechanical wear.
Corrosion is caused by various acids, acidic compounds present in the peat, and also be various micro-organisms. Wear, on the other hand, is caused by the constituents of the ash present in the peat, such as various hard SiO.sub.2, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 crystals as well as sand and stones carried along by the peat. The wear caused by these hard particles is erosive and abrasive by nature. In addition to peat, coal and similar boiler plants, heavy wear occurs also in so-called fluidized-bed furnaces, in which sand-like circulating-dust suspension causes erosive wear of the wall pipes.
Several different methods have been used for preventing the problems of corrosion and wear described above. For example, parts of peat plants have been painted, welded and also thermally sprayed. An advantage of welding is that it produces coatings which are highly resistant to corrosion and erosion. Its disadvantages include deformations due to welding and the fact that, for example it is impossible to coat boiler walls in a short period of time. Furthermore, it is very difficult to produce thin coatings by welding. Painting and other corresponding coating methods can be used when the operating temperature is low and when erosive wear and abrasive wear are not significant.
When it is necessary to coat large surfaces which are exposed to both corrosion and erosion, the best method is thermal spraying. For example, soda-ash boilers have been coated by wire spraying for several years. For the wire spraying of sode-ash boilers there have been developed filler materials which resist corrosion quite well, but their resistance to erosion is not yet good. The reason is that materials resistant to erosion and abrasion are hard, a fact which prevents their being drawn into wire. Materials which are resistant to corrosion, erosion and abrasion can be prepared into powders, and so they are suitable and also used for welding and plasma spraying; also powder spraying is possible.
As was noted above, the use of welding is limited because of deformation and the slowness of the method, and, on the other hand, plasma spraying has a disadvantage in its high price. Powder spraying, for its part, has a disadvantage in that its adherence to the base material is in general poor and the porosity of the coating is high, a factor which promotes corrosion and weakens conduction of heat. Arc spraying has had the same disadvantage as wire spraying; owing to their great hardness it is not possible to roll wire from suitable materials.